In recent years, flat panel displays such as liquid crystal displays (LCDs) have been widespread. In a display device such as a flat panel display, in pixels arranged in the row direction and the column direction, a transistor serving as a switching element, a liquid crystal element electrically connected to the transistor, and a capacitor connected to the liquid crystal element in parallel are provided.
As a semiconductor material for forming a semiconductor film of the transistor, a silicon semiconductor such as amorphous silicon or polysilicon is generally used.
Metal oxides having semiconductor characteristics (hereinafter referred to as oxide semiconductors) are semiconductor materials which can be used for semiconductor films in transistors. For example, techniques for forming transistors using zinc oxide or an In—Ga—Zn-based oxide semiconductor are disclosed (see Patent Document 1 and Patent Document 2). Oxide semiconductors are used for, in addition to a display device, a variety of devices such as a memory and a CPU (see Patent Document 3).